English: Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (P<10 days). The close proximity to their stars and high surface-atmosphere temperatures resulted in their informal name "hot Jupiters".Hot Jupiters are the easiest extrasolar planets to detect via the radial-velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid compared to those of other known types of planets. One of the best-known hot Jupiters is 51 Pegasi b.Though there is diversity among hot Jupiters, they do share some common properties.Their defining characteristics are their large masses and short orbital periods, spanning 0.36–11.8 Jupiter masses and 1.3–111 Earth days. The mass cannot be greater than approximately 13.6 Jupiter masses because then the pressure and temperature inside the planet would be high enough to cause deuterium fusion, and the planet would be a brown dwarf.Most have nearly circular orbits (low eccentricities). It is thought that their orbits are circularized by perturbations from nearby stars or tidal forces. Whether they remain in these circular orbits for long periods of time or collide with their host stars depends on the coupling of their orbital and physical evolution, which are related through the dissipation of energy and tidal deformation.Many have unusually low densities. The lowest one measured thus far is that of TrES-4b at 0.222 g/cm3. The large radii of hot Jupiters are not yet fully understood but it is thought that the expanded envelopes can be attributed to high stellar irradiation, high atmospheric opacities, possible internal energy sources, and orbits close enough to their stars for the outer layers of the planets to exceed their Roche limit and be pulled further outward.Usually they are tidally locked, with one side always facing its host star.They are likely to have extreme and exotic atmospheres due to their short periods, relatively long days, and tidal locking.Atmospheric dynamics models predict strong vertical stratification with intense winds and super-rotating equatorial jets driven by radiative forcing and the transfer of heat and momentum. Recent models also predict a variety of storms (vortices) that can mix their atmospheres and transport hot and cold regions of gas.The day-night temperature difference at the photosphere is predicted to be substantial, approximately 500 K for a model based on HD 209458b.They appear to be more common around F- and G-type stars and less so around K-type stars. Hot Jupiters around red dwarfs are very rare. Generalizations about the distribution of these planets must take into account the various observational biases, but in general their prevalence decreases exponentially as a function of the absolute stellar magnitude.There are three schools of thought regarding the possible origin of hot Jupiters. One possibility is that they were formed in-situ at the distances at which they are currently observed. Another possibility is that they were formed at a distance but later migrated inward. Such a shift in position might occur due to interactions with gas and dust during the solar nebula phase. It might also occur as a result of a close encounter with another large object destabilizing a Jupiter's orbit.